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Old 06-09-2015, 10:15 PM   #1
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using a UPS as a power inverter on a food truck

It's me again. I may have mentioned this in another thread. I ran across the idea of using a discarded UPS as a power inverter. The idea is that people often opt to replace a UPS rather than incurring the cost of new batteries. You use a car battery connected to the alternator on the vehicle with a circuit in between to keep from draining the battery under the hood.

Quoting the poster on another forum:
"We use a second 12v battery with a Schottky battery isolator hooked to our alternator, this allows the truck system to charge the second battery without discharging the truck engine battery. We then take an old UPS and use it as the inverter to run a portion of the electrics and 12v led lighting.
You can google ups as inverter to get some info on how to use the ups as an inverter. The isolator diodes are readily available, used in a lot of big car stereo setups.
This seems like overkill, but has proved handy to keep things running and lighting available during setup and changeovers, and its really not expensive, the UPS was free, the schottky diode and heat sink about $40 and a second 12V truck battery." (Generator/Inverter, staying powered up? Food Truck)

I wanted to run this by you good people to see what you think. I plan to experiment with this soon if I can find an old UPS I think I still have or if my brother finds one at his work. I am looking on Craigslist as well. I will keep everyone updated as this progresses. I appreciate your insight.
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Old 06-09-2015, 11:24 PM   #2
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Please keep us posted.

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Old 06-09-2015, 11:26 PM   #3
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I have run multiple computers for many hours using this method, run off a deep cycle marine battery, very impressed with the results. Now with solar so cheap, I would entertain this again.
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Old 06-10-2015, 03:47 PM   #4
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I did manage to find my UPS unit and the battery inside is indeed dead. To try this out before buying an additional battery, I connected the bus battery under the hood to the terminals that the UPS battery was using. I plugged the machine to one of the battery backup receptacles and it did not turn on. The UPS I was using is only rated at 230W. Does that make a difference if I am using an external battery or a battery bank? Maybe the 230W is a limitation of the circuitry and not a limitation of the battery that is installed at the factory. Of that is the case, I will need a 1000W - 2000W minimum UPS. Not sure if one battery in addition to the battery under the hood would be enough. I am figuring 700W with some headroom for the ice shaver and not sure how much for the chest freezer and cash register along with any fans.
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Old 06-10-2015, 08:31 PM   #5
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. . . Maybe the 230W is a limitation of the circuitry and not a limitation of the battery that is installed at the factory. . .
That is almost always the case with power electronics. The number and size of transistors or similar devices, the connecting wires, etc all affect how much current the electronics can pass. By "right sizing" the electronics, manufacturers can keep costs down, and market share or profits up.
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Old 06-11-2015, 12:42 AM   #6
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Be aware that many if not most UPSes put out modified sine wave which is quite unkind to motors such as your fans and chest freezer. Further, cheaper UPSes will literally cook themselves at duty cycles outside of their original range.
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Old 06-11-2015, 07:14 AM   #7
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Be aware that many if not most UPSes put out modified sine wave which is quite unkind to motors such as your fans and chest freezer.
Never really understood the sine wave thing, can you explain that? Why is it unkind?
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Old 06-11-2015, 07:36 AM   #8
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A modified sine wave is what's called a square wave, it plateaus on each side of zero, and also "rests" before continuing. A true/pure sine wave is a continuous alternating voltage (positive and negative) wave. Digital chipsets read the voltage and base their time on when the voltage reaches 0.

You can see the differences here-


And this article from Xantrex goes into it further, in easy to understand language.
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Old 06-11-2015, 07:41 AM   #9
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And this article from Xantrex goes into it further, in easy to understand language.
meh, the article does not come close to answering my question, but does say MSW is ok for motors?
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Old 06-11-2015, 10:04 AM   #10
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I always understood it as anything other than pure sine wave is dirty electricity and it will prematurely age some devices, some devices won't even start.
UPS's are designed to sit virtually unused for years so if you start using one constantly it may not last long, same concept as "duty cycle" mentioned above
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Old 06-11-2015, 10:16 AM   #11
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OK...I'll ask. Other than big, brown trucks...what are "UPS's"?
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Old 06-11-2015, 10:26 AM   #12
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The explanation disappears down a rat-hole of electrical theory pretty fast.. The "pure sine" power is just that, a single sine wave at 60 Hz frequency. The "modified sine" aka multi-level square wave is equivalent to the sum of a great many sine waves at various frequencies (see Fourier Series). The biggest component is still a 60 Hz sine, but there are many others too. It's the presence of all those other frequencies that can cause trouble because the device may respond to each of those frequencies in different ways. For the most part devices will deal with this, definitely some better than others. For example it's common for devices to run hotter than they normally would when powered with MSW. Also motors may make more noise when fed MSW.
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Old 06-11-2015, 11:16 AM   #13
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The explanation disappears down a rat-hole of electrical theory pretty fast..
Even I had questionable looks on my face in the Electronics Principles section of the Air Force's 3D1 (Cyber Systems Support) tech school

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OK...I'll ask. Other than big, brown trucks...what are "UPS's"?
Uninterruptible Power Supply. They're supposed to provide enough power to save and shut stuff down in the event of commercial power issues.
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Old 06-11-2015, 01:15 PM   #14
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just finished having lunch with my buddy, the electrical engineer. When asked about the good vs. bad, and why, he seemed to think it was hype. When asked what types of appliances or electronics might be negatively affected, he shrugged and said, "MAYBE LEDs"?

Beginning to think this is simply a sales tactic from the inverter industry? As in, "this is much better, that is why it costs more, and if you don't buy this, you will ruin everything you plug into it"
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Old 06-11-2015, 03:06 PM   #15
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The Wynns (who fulltime in a commercial RV) weigh in-
Best Inverter for an RV – Pure vs Modified and Watts
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Old 06-11-2015, 03:40 PM   #16
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The Wynns (who fulltime in a commercial RV) weigh in-
]

ahh yes.... The Shills, uh, I mean The Winns, well at least they acknowledge the sponsorship


I love The Winns, but they provide essentially the same remarks - buy it cuz it is betterer. The only interesting thing they said was "some items will not run on MSW" - and I would like to know what those items are?

Also, what items would be damaged? Of course it would be the expensive kind, otherwise, how could you justify the added costs?

Mr. Shill even says at the beginning of the video "without putting you to sleep with some super-technical nerd-out session...." and then continues by showing more standard waveform graphics, avoiding ANY real info....

But I still want the nerd out session.
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Old 06-11-2015, 04:46 PM   #17
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I think "harmonic distortion" is name of the phenomenon of interest. Also known as "total harmonic distortion" or THD. Here are two articles on that topic: http://www.aptsources.com/resources/...Distortion.pdf Effects of Harmonics on Power Systems | Power Quality content from Electrical Construction & Maintenance (EC&M) Magazine

The first is from a company who sells power sources. They appear to be useful for test lab or other industrial applications. The other is directed to plant managers and other commercial/industrial maintenance type people. These are written from the perspective that some kinds of loads introduce distortion back into the mains that feed them, and also into other equipment connected to the mains. I think it's appropriate to consider the modified sine inverter to be a power source with high THD. Those articles explain how high THD can negatively affect some kinds of equipment.

While searching that out I ran across a reminder that some battery chargers specifically indicate they should not be operated on a MSW supply, and Reports On The Internet (those are always reliable, right??) suggest some tool battery chargers run on MSW may ruin the batteries and/or the charger. In particular, those which are marked "hazardous voltages may be present at the charger terminals" -- but I STILL haven't found any explanation as to WHY this type doesn't play nice with MSW. That's annoying me. This post on candlepowerforums gets close, but from reading just that thread I can't determine to what degree that author might/might not be an expert and in any case there isn't a great level of detail given in the theory.

Power Stream has an interesting inverter FAQ on their site Inverter frequently asked questions.
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Old 06-11-2015, 09:48 PM   #18
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ding ding ding, we have a winner

3 for 3 articles, and now I think I may understand the importance of "clean power" (still reading, the 3rd time now )

thanks family wagon
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Old 06-11-2015, 11:40 PM   #19
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That is almost always the case with power electronics. The number and size of transistors or similar devices, the connecting wires, etc all affect how much current the electronics can pass. By "right sizing" the electronics, manufacturers can keep costs down, and market share or profits up.
Good to know. Thank you for your help!

Quote:
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Be aware that many if not most UPSes put out modified sine wave which is quite unkind to motors such as your fans and chest freezer. Further, cheaper UPSes will literally cook themselves at duty cycles outside of their original range.
I did think about the modified sine wave issue. I see that some UPS units and inverters have true sine wave. They are usually exponentially more expensive. I did not consider the duty cycle.

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I always understood it as anything other than pure sine wave is dirty electricity and it will prematurely age some devices, some devices won't even start.
UPS's are designed to sit virtually unused for years so if you start using one constantly it may not last long, same concept as "duty cycle" mentioned above
Thank you for your insight. I wonder how much they would last as compared to their usual lifespan.
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Old 06-12-2015, 01:12 AM   #20
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I run my 20 volt Dewalt tool battery chargers off MSW inverters in the truck all winter. No issues so far.

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